1. Field of the Invention
The present invention relates to a power supply for switching operation, an electronic apparatus including the same, and a method of controlling the same. More specifically, this invention relates to an increase in response speed in a power supply for switching operation which can change an output voltage value to a plurality of values.
2. Description of the Related Art
A printer which prints information such as a desired character or image on a sheet-like print medium such as paper or a film is known as an information output apparatus for a wordprocessor, personal computer, facsimile apparatus, and the like.
Although various methods are known as the printing method of this type of printer, an inkjet method has recently received a great deal of attention because it can perform non-contact printing on a print medium such as paper, can easily print in color, and is quiet. As an ink-jet arrangement, a serial printing arrangement is generally widely used in terms of low cost and easy downsizing. In this arrangement, a carriage supports a printhead for discharging ink in accordance with desired print information. The printhead prints while reciprocally scanning in a direction perpendicular to the conveyance direction of a print medium such as paper.
Recently, personal computers, digital cameras, and the like are commonplace. An application capable of printing out photographs in accordance with a user's request is also widely used along with the increasing use of digital cameras and the like.
As image input devices such as digital cameras attain higher performance and larger process capacities, a printer serving as an output apparatus needs to achieve higher image quality and higher resolution. To meet this demand, there have been proposed many high-quality printers capable of high-resolution outputs, like outputs on photographic printing paper.
An inkjet printer having the above features is widely used as a printer capable of obtaining a high-quality output at low running cost. Recently, the inkjet printer tends to increase the density of nozzles serving as printing elements and increase the number of nozzles in order to increase the printout resolution while maintaining the print speed.
The inkjet printer generally comprises a discharged pressure generation source including a heater and piezoelectric element in one-to-one correspondence with each nozzle. Increasing the nozzle density and the number of nozzles puts a heavier load on the power supply which supplies power to the discharged pressure generation source including a heater and piezoelectric element.
If the load on the power supply increases, the ink discharge performance varies due to a driving voltage drop. This problem becomes serious particularly when the number of concurrently driven nozzles changes in accordance with print data. For example, if the ink discharge amount and discharge position precision change between printing of only one dot and simultaneous printing of a plurality of dots, the print result reflects these differences as quality deterioration of an output image. This problem occurs not only in the inkjet printer but also in a printing apparatus which adopts another printing method using many printing elements. A change of the driving voltage depending on the number of concurrently driven nozzles influences the power supply capacity, the wiring resistance from the power supply to the discharged pressure generation source, the common impedance and the like.
Along with increases in the number of nozzles and the print speed, the nozzle temperature tends to greatly rise due to remaining heat of the heater after printing. The temperature rise of the nozzle changes ink bubbling, changes the ink discharge amount and discharge position precision, and causes quality deterioration.
To solve the above problems and stably print an image, the driving voltage must be regulated in accordance with the number of concurrently driven printing elements. The inkjet printing apparatus must adopt a measure to regulate the driving voltage to be applied to the heater in consideration of even an increase in nozzle temperature, and stabilize the ink discharge amount and discharge position precision.
To solve the above problems, Japanese Patent Publication Laid Open No. 2001-225457 describes the arrangement of a plurality of driving voltage sources which branch from the same stabilized power supply circuit and have different voltages in a printing apparatus which prints on a print medium by a printhead having a plurality of printing elements. This arrangement can obtain a stable print result even upon a change of the number of concurrently driven printing elements, by selecting an appropriate driving voltage source in accordance with the number of concurrently driven printing elements.
Japanese Patent Publication Laid Open No. 2003-259629 describes feedback control based on the difference (error) between the actual output voltage value and a targeted output voltage value by using a digital control circuit in the control unit of a power supply for switching operation. This arrangement allows controlling an actual output voltage value to follow a targeted output voltage value at high precision and high speed.
However, in the method described in Japanese Patent Publication Laid Open No. 2001-225457 if many driving voltage sources of different voltages are used, this causes to increase the circuit scale and cost. It is, therefore, difficult to finely regulate (set) the driving voltage in consideration of the circuit scale and cost. On the other hand, recent inkjet printers are achieving small ink droplets to be discharged in printing. To stabilize ink discharge in picoliter (pl) order, it is preferable to finely regulate the driving voltage.
According to the method described in Japanese Patent Publication Laid Open No. 2003-259629, the response speed of the output voltage basically depends on the feedback response speed of the switching circuit. To attain both high response speed and stable voltage control, the switching frequency must be increased, which requires high efficiency of the power supply for switching operation. For example, the inkjet printer must regulate the voltage by about 1 V within a short time (e.g., about 10 μsec) in order to regulate the driving voltage of the printhead in correspondence with ink discharge control.
A power supply for switching operation for supplying power to a CPU or DSP generally switches the output voltage using a very high switching frequency in order to reduce power consumption. The printhead driving voltage of the inkjet printer or the like is, e.g., 20 V. The voltage applied to the CPU or DSP is, e.g., 3.3 V. The printhead driving voltage and the voltage applied to the CPU and DSP are different by one order of magnitude of the voltage value. Increasing the switching frequency increases power loss in the power supply and generates a large amount of heat in the power supply.
Attaining both high response speed and high efficiency is a subject common to a power supply for switching operation having a digital control unit as disclosed in Japanese Patent Publication Laid Open No. 2003-259629, and to many other power supplies for switching operation using feedback control whose control frequency is determined by the switching frequency.